Stainless steels



July 28, 1970 N. M. PARIKH I STAINLESS STEELS 5 Sheets-Sheet l FiledJan.

INVENTOR. NIRANJAN M. PARIKH July 28,1970 N. M. PARlKH STAINLESS STEELS3 Sheets-Sheet 2 Filed Jan. 5, 1966 FIG INVENTOR. NIRANJAN M. PARIKHJuly 28, 1970 N. M. PARlKH STAINLESS STEELS 5 Sheets-Sheet 5 Filed Jan.5, 1966 FIG INVENTOR. NIRANJAN M. PARIKH United States Patent 3,522,020STAINLESS STEELS Niranjan M. Parikh, Chicago, Ill., assignor to IITResearch Institute, Chicago, 11]., a not-for-profit corporation ofIllinois Filed Jan. 3, 1966, Ser. No. 518,130 Int. Cl. B22f 3/12 US. Cl.29-182.5 1 Claim ABSTRACT OF THE DISCLOSURE Fully dense stainless steelalloys characterized by having a microstructure exhibiting asubstantially uniformly dispersed carbide phase of particle sizeessentially less than on micron, are produced from pre-alloyed powdersby rapidly quenching an atomized alloy charge and subjecting thesolidified particles in a suitable container to the step of hotconsolidation to produce fully dense metal stock directly fromprealloyed powder.

The present invention relates to a novel stainless steel of uniquemicrostructure and to a process whereby such steel may be readily made.More especially my invention is based upon my discovery that by the useof a molten metal atomization process to yield alloy powders followed bythe consolidation thereof as herein taught, there results very desirablestructural alloys characterized by such microstructure.

At the present time there are commercially available cast stainlesssteels which are designated as Ph -7 M0 by the trade. Such alloys havethe following composition:

Carbon0.09 max. wt. percent Manganesel.0 max. Silicon-1.0 max.Chromiuml4-16 Nickel-6.57.75 Molybdenum--2-3 Aluminum0.751.5 Sulfur0.03max. Ironbalance The alloy identified as Ph 15-7 Mo is one of a numberof alloys, both iron base and other base, which in present technologyare referred to as superalloys. These are heat-resistant, high strengthmaterials which are quite well suited to meet a number of the extremedemands now placed upon modern structural materials. However, until themaking of my invention the manufacture of various structures out of suchsuperalloys has been a fairly expensive process involving the steps ofcasting, rolling, forging or extrusion and subsequent machining. Yeteven despite the expense of the materials, the present requirements ofhigh temperature uses is pressing the operating capabilities of thesuperalloys.

To meet this dual problem of expense and the need for improvedsuperalloys I have discovered a new method of their fabrication,especially for Ph 15-7 Mo, which not only is considerably moreeconomically feasible than the processes of fabrication of the priorart, but of even greater importance I discover that such alloys made asherein taught are characterized by a unique, fine-grainedmicrostructure, which leads to improved physical and mechanicalproperties therein.

I accomplish these beneficial results by first atomizing a melt of theappropriate alloying charge to produce powders thereof followed byconsolidation of the powders into solid stock. In carrying out theatomization step due care must be given in order to prevent the pick-upof embrittling impurities such as oxygen and nitrogen.

Patented July 28, 1970 Powder consolidation is preferably accomplishedby hot pressing the material while again protecting it from picking upundesirable embrittling impurities and in some cases following the hotpressing by an extrusion step or the like to yield the desiredend-product structural shape.

The improved properties of the present stainless steels stem from thenovel microstructure which is obtained by my process. Rather than thecoarsened grain structure found in commercial alloys of similarcompositions, I find that in the alloys made as herein taught there isan extremely fine-grained microstructure and wherein the hardening phaseis of exceptionally small grain sizeusually smaller than 1 micron-andfurthermore that such hardening phase is substantially uniformlydispersed throughout the material.

Accordingly, a primary object of my invention is to provide a newstainless steel of the type designated Ph l57 M0 which is characterizedby a fine-grained microstructure and wherein the hardening phase is of asize essentially less than 1 micron and wherein said hardening phase isuniformly dispersed through the major phase matrix.

Another object of my invention is to provide a process of making suchstainless steels of unique microstructure which includes the steps ofatomizing to form powders thereof followed by the consolidation of saidpowders into metal stock.

These and other objects, features and advantages of my invention willbecome apparent to those skilled in this particular art from thefollowing detailed disclosure thereof and the accompanying drawings inwhich:

FIG. 1 schematically illustrates an atomizing chamber for use in thepractice of the present invention;

FIG. 2 schematically illustrates atomizing apparatus for use herewith;

FIG. 3 is a photomicrograph of a commercially available Ph 15-7 Mo alloyat a magnification of x; and

FIG. 4 is a photomicrograph of an alloy made as herein taught at amagnification of 320x In order to prepare the present alloys I startwith an atomization process of an appropriate molten alloy charge. Theapparatus for this is schematically illustrated in FIGS. 1 and 2, withit of course being understood that other similar apparatus may belikewise employed. In such process one may either begin with standardcast alloy materials, if they are available, as the initial charge, orthe charge may be prepared from its elemental constituents. The chargeof the materials was first weighed up and then melted under an argonatmosphere in a high frequency induction unit. The heating element isnot shown in the drawings and obviously a number of equivalent meltingmeans can be used. It is necessary to melt under the protection of inertatmosphere or vacuum in order to prevent oxidation and other undesirablecontamination of the melt.

I weighed up and then melted 5 pound charges of the composition set outabove of alloys Ph l5-7 Mo.

The molten metal charge is then poured into a preheated (to preventchilling) magnesia-lined pouring cup 26 placed at the top of theatomizing chamber 22. The melt passes first through a hole 21 at thebottom of the pouring cup through a refractory-lined nozzle 27. By thismeans the molten charge enters the atomizing chamber 22. As the chargeemerges from the nozzle 27 the molten stream is first broken up intofine particles and then quickly quenched by a high pressure inert gasstream entering the chamber 22 through the gas inlet port 23. In orderto assure rapid quenching of the molten atomized -metal powders Iprovide a water reservoir 24 at the bottom of the chamber which mayoperate in conjunc- 3 tion with the atomizing stream to rapidly quenchthe particles. At the bottom of the chamber I provide a capped opening25 for metal powder removal. To complete the structure there is alsoprovided a gas exit port 28.

After the alloy powders are formed they are removed from the atomizingchamber, separated from the water and dried. Drying may be most simplyaccomplished by limited heating to drive off the water.

The alloy powders were then canned in mild steel encapsulating material.Air was evacuated from the cans and they were then sealed. Such canswere then hot-pressed to consolidate the atomized powders. In carryingout such sintering it should be noted that great care should be taken toavoid melting the particles and at the same time contamination of theresulting sintered billets should also be avoided. The stainless steelalloys of my invention were sintered readily under an argon atmosphereto avoid the loss of volatile chrominurn among other things. Suitablesintering temperatures are between the range of 2550 and 2690 F.

I find it is preferable to hot-press the powders rather than merelysintering them. To accomplish this the alloy powders are sealed in 0.040inch thick stainless steel cans, heated at 2190 F. and then hammerforged without side restraint. The height reduction was about 60%.Although the edges of the forged pieces were somewhat porous andcracked, I found that at slightly below such surfaces the forged pieceswere 100% dense.

The most preferable mode of consolidating powders is by extrusion. Greencompacts of the atomized powders, 1% inch high by 1 inch in diameterwere sealed in close fitting mild steel cans under vacuum. These werethen heated to 2200 F. in a resistance furnace, then dropped quicklyinto an extrusion die and rapidly loaded to 40 tons per square inch.Such load was maintained for 15 seconds, then the in plate was removedand the compact was pushed out of the die. 100% dense structures werethus obtained.

The advantages of my invention may readily be seen by comparing FIGS. 3and 4 hereof. FIG. 3 is a photomicrograph of commercially available Ph15-7 Mo and the coarse martensitic-type structure is immediatelyapparent. In distinction to this the fine-grained structure illustratedin FIG. 4 of a T-section extruded as herein taught and the advantagesthereof will be immediately apparent to those skilled in this art.

In fabricating the present alloys it should be cautioned that the oxygencontent should be maintained as low as possible, preferably below 1000parts per million and the nitrogen content below 100 parts per million.

It should be understood that various modifications and variations may beeffected without departing from the spirit or scope of the novelconcepts of my invention.

I claim as my invention:

1. A fully dense consolidated powder alloy comprising: from 14 to 16%chromium; from 6.5 to 7.75% nickel; from 2 to 3% molybdenum; from 0.75to 1.5% aluminum; carbon up to 0.09% maximum; magnanese up to 1%;silicon up to 1%; sulfur maximum of 0.03%, balance iron, said alloybeing characterized by a substantially uniformly dispersed hardeningphase in the fine-grained major phase matrix, said hardening phase beingessentially of a size less than a few microns and the said fine-grainedmatrix phase being less than about microns.

References Cited UNITED STATES PATENTS 2,958,618 11/1960 Allen 124 XR2,958,617 11/1960 Perry 75124 XR 3,071,463 1/1963 Hausner 75213 XR3,151,978 10/1964 Perry 75-124 OTHER REFERENCES Goetzel, Treatise onPowder Metallurgy, vol. I, pp. 15, 1949, Interscience Publishers, Inc.,N.Y.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant ExaminerUS. Cl. X.R.

